Overhead cables of this kind, especially in the form of double-conductor cables, are used as telephone lines. In the past, telephone lines of this kind have been used in areas where telephone subscribers are located relatively far from a central exchange, or from the terminal of a subterranean telephone-cable system, and it would be too expensive to lay subterranean telephone lines running to these subscribers, because of the distance involved and the cost of providing a cable-tunnel for carrying only one or a few lines. In these known telephone cables for overhead lines, mainly steel wires have been used as the tension-bearing means, usually in the form of tinned copper wire and provided for signal transmission, constituting the individual conductors of the cable. In these known telephone lines, each of the two conductors had a polyethylene casing and an overlying polyamide casing, and were joined by means of an integral bridge made of the same polyamide between the two polyamide casings. These known telephone lines, however, have a major disadvantage, namely that the steel wires provided in the individual conductors as the tension-relieving means result in the conductors being substantially more liable to corrosion than conductors consisting exclusively of copper wires. For example, a series of failures of these mixed wire telephone lines was caused by leaks developing in the course of time in the polyethylene casing enclosing the conductors, for example at kinks or at locations of high mechanical alternating stress, and allowing water to penetrate into the conductors. This led eventually to failure of the conductors by corrosion at such locations. For the purpose of overcoming this disadvantage in known telephone lines, attempts were first made to reduce the corrodibility of the mixed wire conductors, to approximately that of conductors made exclusively of copper wire, by tinning not only the copper wires, but also the steel wires. Although this resulted in a certain decrease in the corrodibility of mixed wire conductors, the decrease was not down to the level of conductors made entirely of copper wires, because it was found impossible to produce, on the steel wire a coating of tin which would completely exclude water. The theoretical possibility of providing the steel and copper wires with completely impenetrable coatings of tin to achieve complete resistance to corrosion equal to that of tinned copper wires was in practice far from being achieved.
In other types of cables than, it is known to replace the tension-bearing steel wires within the conductors with fibres, or bundles of fibres, of high-strength non-metallic materials such as glass fibres, arranged within the cable-casing in the form of longitudinal reinforcing elements. The use of such non-metallic materials as tension-bearing mens naturally eliminates the problem of corrodibility arising with the use of steel wires. However, the arrangement used in known cables, namely to arrange within the cable casing the high-strength fibres in parallel with the axis of the cable and in the form of a layer of fibres, or of a bundle of fibres, distributed uniformly around the periphery of the conductor could not be transferred to overhead cables of the present type, since the fibre reinforcement of the cable-casing made the flexural strength or stiffness of the cable too high for overhead cables. The main reason for this is where the fibres in these known cables run parallel with the cable axis, any bending of the cable requires the fibres on the outside of the bend to stretch, but high-strength fibres resist this because of their resistance to elongation. Since overhead cables are subjected, at least in the vicinity of their suspension points, to relatively high and constantly alternating flexural stress, high flexural stiffness would very soon cause the fibres in areas of high flexural stress to break, thus eliminating the tension-bearing of the cable and leading sooner or later to complete breakage of the overhead cable, for example under conditions of very heavy loading, such as a storm. Now it is known, in the case of overhead telephone cables, that flexural stiffness and the consequences thereof, in the form of broken cables, caused by arranging the tension-relieving means in parallel with the axis of the cable, may be avoided by stranding the individual mixed wire conductors. The result of stranding, however, is that the overall length of the wires within the individual conductors, because of their spiral configuration due to stranding, is greater than the overhead cable itself, which means that the cable would be capable of undergoing elongation without actually stretching the wires, if it were possible for the wires to change from a spiral configuration to one coinciding with the axis of the cable. In the case of the overhead telephone cables, this is impossible because the wires enclosed within the individual casings position themselves mutually in each conductor, thus making impossible any displacement of the wires towards the axis of the cable under tensile loading. However, if in these overhead cables, the steel wires provided for tension-carrying effect were to be replaced simply by bundles of synthetic fibres running parallel with each other like cords, it would be quite possible for individual fibres in these bundles to shift towards the centre of the axis under tension, since the individual fibres in the bundles are not fixed within the conductor by the copper wires. This may be seen, for example in FIG. 1, if it is assumed that the unhatched circles are either steel wires or bundles of fibres consisting of individual fibres running parallel with each other, and that the hatched circles are copper wires. Where steel wires are used, the copper and steel wires position themselves mutually and this cannot be altered by loading the cable in tension; on the other hand, in the case of bundles consisting of individual fibres, the fibres in the three outer bundles can easily shift towards the centre. Initially all the six interstices grouped around the center bundle of fibres would be filled, whereupon the copper wires would be forced outwardly until the fibres in the outer bundles regroup themselves around the central bundle in generally layer fashion. Simultaneously with this regrouping, which would obviously take place only when the cable under tension, the cable would now lengthen in accordance with the now smaller average diameter of the spiral configuration of the three outer bundles of fibres, and the central bundles, which would be unable to lengthen and thus be subjected to the full tensile load and break, whereas the copper wires, of only relatively low tensile strength and thus capable of stretching, would stretch accordingly. Thus in spite of the exceptional resistance of the synthetic fibres to elongation under tension, the cable would stretch to the length attributable to the aforesaid regrouping. Therefore, the result of merely replacing the steel wires in the overhead cable of the type in question by bundles of synthetic fibres is the loss of stretch-resistance, and since resistance to stretching is one of the main requirements of an overhead cable, it is impossible, in the case of the known overhead cable, to replace the steel wires with high-strength synthetic fibres, and thus to overcome the corrosion problems mentioned hereinbefore, without taking special precautions.